The compression ratio of a reciprocating internal combustion engine c refers to the ratio of the volume of the cylinder chamber as a whole to the volume of the compression chamber. By increasing the compression ratio, the efficiency of the reciprocating internal combustion engine can be increased and therefore the fuel consumption can, on the whole, be reduced. However, in the case of spark-ignited reciprocating internal combustion engines, the knocking tendency thereof increases with the increase in the compression ratio in full-load operation. Knocking is an uncontrolled self-ignition of the fuel/air mixture.
In contrast, the compression ratio could be increased in partial-load operation, in which the cylinder fill is lower, to improve the corresponding partial-load efficiency without causing the above-mentioned knocking to occur. As a result, it is expedient to operate the reciprocating internal combustion engine with a relatively high compression ratio in partial-load operation and with a comparatively reduced compression ratio in full-load operation.
Changing the compression ratio is moreover particularly advantageous for supercharged reciprocating internal combustion engines with spark ignition since, on the whole, these require a low compression ratio for supercharging, wherein the compression should be increased to improve the thermodynamic efficiency in unfavorable areas of a corresponding engine map. Moreover, the option exists of generally altering the compression ratio depending on further operating parameters of the reciprocating internal combustion engine, such as driving states of the vehicle, operating points of the internal combustion engine, signals of a knock sensor, exhaust gas values etc., for example.
Devices which are used to adjust the effective length of the connecting rod are known, inter alia, from the prior art. These are devices which are used to alter the spacing between a connecting rod bearing eye of the connecting rod, which is arranged on a crankpin of a crankshaft, and a piston pin bearing arranged in a connecting rod eye of the connecting rod. In this case, the adjustment can take place at the connecting rod eye receiving the piston pin bearing or at the connecting rod bearing eye mounted on the crankpin, whereby the position of the piston relative to the crankpin is altered in each case.
A device for altering the compression ratio of a reciprocating internal combustion engine of the generic type is known from DE 10 2012 112 461 A1. According to this, a connecting rod which is divided in the region of its connecting rod eye receives, via the bore of this connecting rod eye, an eccentric body on which a two-arm eccentric lever acts and in which a center point of a piston pin bore is arranged offset relative to a center point of the connecting rod eye. The eccentric body is guided in the bore of the connecting rod eye by a cylindrical outer circumferential surface and is designed as a slide bearing. Rotational movements of the eccentric body in the bore of the connecting rod eye take place automatically. These are triggered by the effect of inertial and gas forces of the cylinder unit, wherein the effective directions thereof change constantly during its working process. At its ends, the eccentric lever is furthermore connected via a pivot bearing in each case to a piston rod receiving the control piston, wherein the control pistons are guided in control cylinders.
A rotational movement of the eccentric body under the inertial and gas forces of the cylinder unit, and consequently a pivotal movement of the eccentric lever in one of the two directions, is enabled in that the pressure chamber of the control cylinder which is located in the direction is without pressure whilst engine oil flows into the pressure chamber of the other control cylinder at the corresponding pressure of the lubricating oil pump. This control is undertaken by a slide valve, designed as a control slide valve, of a switching device which is arranged in the vicinity of the connecting rod eye and is constructed as a switching valve. In this case, the control slide valve is mechanically displaced into one of its two switching positions so that in each case one of the pressure chambers empties without pressure into the oil pan of the reciprocating internal combustion engine and the other pressure chamber is filled with engine oil.
In this case, a housing, described as a sleeve-shaped connecting element, is inserted into a receiving bore extending parallel to the longitudinal center axis of the connecting rod bearing eye, which housing has a first and a second bore, each connected to hydraulic fluid lines, and a vent bore connected to a vent channel. The slide valve, which is longitudinally displaceable in the housing, has a T-shaped groove via which one of the two bores is optionally connected to the vent bore. To adjust the slide valve each of its two switching positions, an actuating pulse, transmitted by means of a preferably electrically actuated control element, is said to be provided.